The overall goal of the studies presented in this proposal is to identify the immunological mechanisms that mediate effective protection from shigellosis following vaccination and natural infection with Shigella. Shigella is a global infection that disseminates rapidly in settings where there is crowding and inadequate sanitation. Given the shortcomings of available public health measures to successfully control this infection, the appearance of drug resistance and concerns about its potential use in bioterrorism, the development of safe and effective vaccines to S. dysenteriae 1, S. flexneri 2a and S. sonnei is urgently needed. However, Shigella vaccine development has been hampered by a considerable lack of information of the specific determinants of protective immunity. Thus, the understanding of the immunological correlates of protection in humans to Shigella spp. is of great importance. Our working hypothesis is that both cell-mediated immunity and antibody responses play a central role in protection of volunteers from shigellosis following immunization with attenuated Shigella vaccine candidates or infection with wild-type Shigella. Specifically, using serum, stool and peripheral blood mononuclear cell specimens obtained from volunteers immunized with attenuated strains of S. dysenteriae, S. sonnei or S. flexneri 2a or challenged with wild-type S. dysenteriae (zlstxA strain), S. sonnei or S. flexneri 2a, we propose to test the following hypotheses: (1) secretion of interferon-gamma, and other cytokines to key Shigella proteins involved in cell invasion (e.g., IpaB, IpaC and IpaD) following immunization or challenge of volunteers with Shigella is mediated by CD4 + T cells and restricted by class II major histocompatibility complex molecules; (2) immunization or challenge of volunteers with Shigella elicits the appearance in circulation of specific cytotoxic T lymphocytes; (3) protective CMI against Shigella depends on a defined set of immunodominant epitopes derived from Shigella antigens; (4) immunization or challenge of volunteers with Shigella elicits the appearance in serum and stools of antibodies directed not only to LPS, but also to key molecules involved in Shigella invasion, e.g., IpaB, IpaC, IpaD and other Shigella proteins and (5) immunization or challenge of volunteers with Shigella elicits the appearance in circulation of (a) specific T and B lymphocytes expressing gut homing molecules (e.g., integrin alpha4beta7)and (b) expanded effector (Teff) and peripheral memory T cell (TEM)pools.